1. Field of the Invention
The present invention relates to continuously variable transmissions, and, more particularly, to control systems and strategies for continuously variable transmissions which include a double acting secondary sheave servo. The system and strategy provides sufficient clamping force at lower transmission ratios, avoids over-pressurizing the secondary servo, uses pressures at or below those found in conventional continuously variable transmission secondary servos, and thereby makes it easier to achieve the desired primary to secondary ratio.
2. Description of the Prior Art
A continuously variable transmission (CVT) utilizes a pair of adjustable or variable pulleys mounted on a pair of shafts, and an endless belt intercoupled therebetween, to transmit torque from an input source, such as an engine, to an output, such as a vehicle driveline. Each pulley has at least one sheave that is axially fixed and another sheave that is axially movable relative to the first. A flexible belt of metal or elastomeric material interconnects the pulleys.
The inner faces of the sheaves of the pulleys are bevelled or chamfered so that movement of the axially displaceable sheave relative to the fixed sheave adjusts the distance between the sheaves and, thus, the effective pulley diameter. The first pulley, or primary pulley, is mounted on a primary or input shaft and is driven by the engine through a torque converter or start clutch. The second pulley, or secondary pulley, is mounted on a secondary or output shaft and the output of the secondary shaft drives the drive train of the vehicle. The drive train is typically connected to the secondary shaft through a clutch. U.S. Pat. No. 4,433,594, entitled "Variable Pulley Transmission", provides further information regarding CVTs and is incorporated herein by reference in terms of background to the present invention.
The primary and secondary variable pulley systems include servo systems for movement of the pulley sheaves. The displaceable sheave of each pulley includes an annular chamber for receiving fluid to move the sheave and thus change the effective pulley diameter. Increasing the fluid in the chamber increases the effective diameter of the pulley. As fluid is exhausted from the chamber, the pulley diameter is decreased. The effective diameter of the primary pulley is moved in one direction as the effective diameter of the secondary pulley is moved in the other direction.
The movement of the sheave of the primary pulley servo regulates the transmission ratio across the CVT. The movement of the sheave of the secondary pulley servo regulates the clamping force on the belt connecting the primary and secondary pulleys. Sufficient clamping force is therefore necessary to prevent damage that might result from slippage of the belt.
The sufficiency of secondary sheave clamping force becomes especially problematic in high torque applications, because certain designs of conventional CVT's that can provide the necessary clamping force will thereby have insufficient output torque or inadequate range of vehicle drive ratios (such a wide range is desirable for providing fuel efficient overdrive ratios). For example, at launch of a vehicle there is a need for high wheel torque, and at a very slow vehicle speed there is a need for a low transmission ratio and high torque. This need is complicated by the low torque supplied by the engine at low engine speeds. To maximize launching torque, a torque converter may be inserted between the engine output and CVT input to multiply the starting torque at launch. This torque converter can then be locked up at high vehicle speeds thereby having no effect at the CVT's high speed transmission ratio. Unfortunately, the torque multiplication provided by the torque converter increases the secondary clamping force required to prevent belt slippage.
The output torque of the CVT and the effective radius of the primary sheave thus are factors determining the necessary clamping force at the secondary sheave to prevent belt slippage. At vehicle launch, the necessary high output torque and small effective radius of the primary sheave requires the secondary servo clamping force to be at its highest.
Another factor determining the necessary clamping force becomes more important after vehicle launch. This is the centrifugal force of the rotating oil within the servo, which increases with the square of the angular velocity of the sheave and the fourth power of the sheave radius, and can cause excessive belt clamping force at high secondary pulley angular velocities. In a single secondary, a balance can is placed typically adjacent to the servo and counteracts this centrifugal force. The balance is typically open to atmosphere and kept filled with oil either through an orifice in the apply servo or from an external source, and therefore only reduces the belt clamping force by way of the centrifugal force of the oil in the balance can.
The centrifugal effects also contribute to transmission inefficiency because such forces can make it difficult or impossible to obtain the required primary to secondary belt clamping force ratio necessary to maintain a desired transmission ratio, and in any event there is the likely result of over-pressurizing the primary and secondary servos. Thus, for example, when a vehicle is at a speed allowing for an overdrive ratio, the secondary clamping force is thereby at a high level, and the primary must be even greater to obtain the clamping force ratio necessary to maintain the desired transmission ratio in overdrive. Moreover, the centrifugal effects also can be problematic at high vehicle speeds when downshifting is required because the centrifugal forces are opposed to attempts at altering the effective secondary pulley diameter required for downshifting.
Thus, the operation of secondary servos, both at launch, during upshift, overdrive, and downshift, in high torque applications necessarily involves high pressures that can cause inefficiency, noise, and mechanical problems to the hydraulic pump providing the source pressure to the system as well as other mechanical difficulties in the system.
Heretofore, efforts have been made to provide sufficient clamping force to a secondary sheave in high torque applications, such as when a torque converter is placed between the engine output and the CVT input. One such effort described in U.S. Ser. No. 07/721,285, filed Jun. 26. 1991, now U.S. Pat. No. 5,180,339 utilizes a double acting secondary servo with a plurality of fluid chambers in order to increase the application of fluid pressure, and a balance cavity between two servo areas, allowing for the required clamping forces necessary to operate the CVT. However, control arrangements in the prior art are not suitable for optimum regulation of such a CVT. The present invention is an attempt to provide a control system and strategy capable of controlling belt clamping by the aforementioned secondary servo to a level that allows for the most efficient operation of the CVT. While control strategies for secondaries are known in the prior art, the present invention utilizes the aforementioned double-acting secondary servo in an attempt to address the above-described problems.